In the patent literature, active noise control in a jet engine is shown in U.S. Pat. No. 4,044,203 to Swinbanks. Swinbanks describes the method of arranging microphones and speakers in both the inlet and exhaust ducts of a jet engine as well as ducts in general such that the resultant output of the speakers will cancel the desired noise without emitting noise itself in the opposite direction. The arrangement also ensures that there is no feedback from the speakers to the microphones.
U.S. Pat. No. 4,815,139 (L. J. Eriksson, et al.) from Col. 1, line 64 through Col. 2, line 22 discusses the possibility of splitting the duct to increase the transverse resonant frequency and thus allow the active noise control system to work at higher frequencies.
U.S. Pat. No. 5,119,902 (Earl R. Geddes) discloses a system for modifying the duct to form an efficient speaker enclosure so that sufficient acoustic power can be applied in a small enough package to provide attenuation in the automobile exhaust system, however not involving the use of flow straighteners.